1. Field of the Invention
The present invention relates to a configuration in which a microphone is attached inside a duct, for use in an active noise reduction device employing an active noise control technique for reducing a noise generated by information processing equipment and the like.
Information processing equipment and the like include ventilating of the inside of the equipment in order to release heat generated inside the equipment to the outside. In recent years, there has been a demand for better office environments with information processing equipment with lower noise levels, so that noise coming out of such equipment need to be reduced. However, noise reduction measures taken for each device component is not enough to achieve this end, and sound absorbing ducts with acoustical material affixed therein are used in exit openings of ventilators. In addition to that, in order to eliminate a noise which is not erased by sound absorbing ducts, an active noise control technique is used for counteracting this noise by using a sound of the same magnitude and a reversed phase.
2. Description of the Prior Art
As shown in FIG. 1, a typical mechanism for releasing inside heat to the outside has duct 2 provided with a fan 1, which generates a current of air for releasing inside heat. One of the methods of reducing noise inside the duct 2 is to paste inside the duct 2 acoustic material 3 such as forming material. This method is mainly concerned with a reduction of high pitched sounds in noise.
As noted above, the acoustic material 3 mainly absorbs high frequency components of noise, and is not suitable for the reduction of low frequency components. An active noise control method is a method of reducing such noise persisting despite the acoustic material 3.
FIG. 2 shows an example of a prior art control system for an active noise reduction device employing the active noise control method. As shown in FIG. 2, the active noise reduction device comprises a noise reduction filter 10, a reversing sound compensation filter 11, a FIR (Finite Impulse Response) filter 12, an arithmetic operation control unit 13, a speaker 14, a noise pick-up microphone 4, and an error pick-up microphone 15.
The noise pick-up microphone 4 is a microphone which is provided on the side of a noise source to pick up noise. The noise reduction filter 10 is a FIR filter which generates a noise reduction signal by using as a reference signal a noise signal picked up by the microphone 4, and which emulates a transfer system A of the noise.
The speaker 14 is used for transforming the noise reduction signal produced by the noise reduction filter 10 into a noise reduction sound, and superimposes this sound on noise coming through the transfer system A of the duct. The error pick-up microphone 15 is a microphone which picks up persisting noise (difference between noise and the noise reduction sound) in order to update filter coefficients of the noise reduction filter 10. A signal detected by the microphone 15 is provided for the arithmetic operation control unit 13.
The FIR filter 12 is a filter for emulating a transfer system B which is constituted by the path from the output of the noise reduction filter 10 to the error signal input of the noise reduction filter 10 via the speaker 14 and the error pick-up microphone 15. The FIR filter 12 is provided in order to factor in the influence of the transfer system B by using such algorithms as Filtered-x, because noise to be reduced cannot be counteracted in the noise reduction device unless the output of the noise reduction filter 10 passes through the transfer system B including the speaker 14.
The arithmetic control unit 13 is a circuit which controls the noise reduction filter 10 by executing arithmetic operations used for updating its filter coefficients on the basis of the output of the FIR filter 12 and a signal picked up by the microphone 15.
The reversing sound compensation filter 11 is a filter which emulates a transfer system C directing upstream in the same path as the transfer system A. In the noise reduction device, the noise reduction sound transmitted by the speaker 14 propagates in the transfer system C directing upstream in the same path as the transfer system A to reach the noise pick-up microphone 4, which obstructs the generation of a proper noise reduction sound by the noise reduction filter 10. In order to negate the effect of this reversing sound which passes through the transfer system C, the noise reduction device emulates the transfer system C to produce a reversing sound signal by passing the noise reduction signal of the noise reduction filter 10 through the reversing sound compensation filter 11, and subtracts this reversing sound signal from the signal picked up by the noise reduction microphone 4.
When employing active noise control, a proper noise control cannot be implemented if a microphone provided in a duct picks up noise generated by a current of air therein. This noise is sounds made by a friction of air with the microphone and, also, made by shaking movement of the microphone blown by an air current. Thus, a proper counter measure for the air current becomes necessary. A counter measure for the air current in the prior art is to plant a microphone in the acoustic material pasted on the inside wall of the duct, thus reducing an effect of the air current on the microphone.
However, keeping pace with recent development of information processing equipment, heat generated inside the equipment has a tendency to increase. Thus, there is a need to increase a heat release capacity of the equipment, which leads to the use of fans having a larger capacity of ventilating. As a result, the velocity of a current passing through a duct becomes higher, which means that an effect of the current on a microphone provided in the duct is also increased. Accordingly, it becomes necessary to develop more advanced technologies for air current counter measures.
Accordingly, there is a need in the fields of active noise reduction devices for a configuration of providing a microphone in a duct which can reduce an effect of an air current on the microphone, and realize a higher noise reduction capacity.